Field of the Invention
The present invention relates to a photoelectric conversion device and an imaging system.
Description of the Related Art
In a photoelectric conversion device in which a plurality of pixels form an imaging surface, so-called global electronic shutter is known in which the start time and the end time of charge accumulation operations are the same in all the pixels within the imaging surface. Japanese Patent Application Laid-Open No. 2013-172204 discloses a photoelectric conversion device having a global electronic shutter function.
Each pixel in the photoelectric conversion device disclosed in Japanese Patent Application Laid-Open No. 2013-172204 includes a photoelectric converter that generates charges, a first transfer switch that transfers charges from the photoelectric converter to a first holding portion, and a second transfer switch that transfers charges from the first holding portion to an FD region. Further, the pixel in the photoelectric conversion device disclosed in Japanese Patent Application Laid-Open No. 2013-172204 includes a third transfer switch that drains charges from the photoelectric converter to an overflow drain. With the same timing that stops draining charges from the photoelectric converter to the overflow drain in all the pixels, it is possible to have the same start time of the exposure period for all the pixels. Further, with the same timing of transferring charges from the photoelectric converter to the first holding portion in all the pixels, it is possible to have the same end time of the exposure period for all the pixels. This allows a global electronic shutter operation to be realized.
Japanese Patent Application Laid-Open No. 2013-172204 discloses that, during the exposure period of the photoelectric converter, the potential barrier of the third transfer switch with respect to charges accumulated in the photoelectric converter is set lower than the potential barrier of the first transfer switch with respect to charges accumulated in the photoelectric converter. This can prevent charges generated during an exposure period of the photoelectric converter from overflowing to the first holding portion side.
With a lower potential barrier of the third transfer switch during the exposure period of the photoelectric converter, however, the amount of signal charges that can be accumulated in the photoelectric converter (saturation charge amount) decreases resulting in a narrower dynamic range of the output signal. On the other hand, with a higher potential barrier of the third transfer switch during the exposure period of the photoelectric converter for increasing the saturation charge amount of the photoelectric converter, leakage of charges into the first holding portion from the photoelectric converter may occur, which may be superimposed as noise on the previous frame signal.